1. Field
The present work relates generally to wireless communications systems, methods, computer program products and devices, and more specifically to techniques for efficient fractional frequency reuse in wireless communications systems.
2. Background
In a Fractional Frequency Reuse (FFR) cellular system, data can be transmitted to each wireless mobile unit over one of many multiple frequency sub-bands in which each sub-band may have a different effective spectral efficiency. Moreover, these effective spectral efficiencies can vary over time due to channel fading. In such systems, it is important to allocate the spectral resources intelligently to the users in order to maximize the net benefit of these resources to all users. In particular, a good scheduling policy will have properties that (1) allocate spectral resources to a user for which the channel gains on some of the frequency bands are close to their peaks, (2) for a given user, allocate resources on frequency bands with higher channel gains, rather than lower channel gains, and (3) ensure fairness between the different users by making sure that no user is deprived of service for an undue amount of time.
For example, consider the operation of a wireless communications network. FIG. 1 illustrates a portion of a wireless communications network that comprises a first wireless base station 110 (designated ENB1) and a second wireless base station 120 (designated ENB2). FIG. 1 also illustrates a first mobile unit 130 (designated UE1) and a second mobile unit 140 (designated UE2) that are located between the first base station 110 and the second base station 120. The first mobile unit 130 is located near an edge (not shown) of the wireless cell that is centered on the first wireless base station 110. The second mobile unit 140 is located near an edge (not shown) of the wireless cell that is centered on the second wireless base station 120.
The mobile unit 130 receives wireless transmissions from (and sends wireless transmissions to) the first wireless base station 110 on wireless link 150. The mobile unit 140 receives wireless transmissions from (and sends wireless transmissions to) the second wireless base station 120 on wireless link 160.
In addition, the mobile unit 130 receives interfering wireless transmissions from the second wireless base station 120. The “out of cell” interference that is received by the mobile unit 130 from the second wireless base station 120 is shown in FIG. 1 as wireless link 170. Similarly, the mobile unit 140 receives interfering wireless transmissions from the first wireless base station 110. The “out of cell” interference that is received by the mobile unit 140 from the first wireless base station 110 is shown in FIG. 1 as wireless link 180. The mobile units (e.g., first mobile unit 130 and second mobile unit 140) that are located near the edges of wireless cells receive significant “out of cell” interference if the wireless base stations (e.g., first wireless base station 110 and second wireless base station 120) transmit at maximum power.
In a Fractional Frequency Reuse (FFR) cellular system the “out of cell” interference can be minimized by employing an FFR method that transmits data to each mobile unit over one of many multiple frequency sub-bands, in which each sub-band may have a different effective spectral efficiency. The FFR method reserves some resources for the “out of cell” mobile units at each of the wireless mobile stations. In general, the available bandwidth (BW) is divided into a number of sub-bands. Each mobile unit is served at a different Signal to Interference/Noise Ratio (SINR) on each sub-band.
FIG. 2 illustrates how bandwidth at a base station can be subdivided into multiple sub-bands in a fractional frequency reuse (FFR) method. The bandwidth at the first wireless base station 110 (ENB1) is represented in FIG. 2 as a rectangle 210. The rectangle 210 may also be referred to as the bandwidth 210. The bandwidth frequency in the diagram increases from the bottom of rectangle 210 to the top of rectangle 210. The bandwidth at the second wireless base station 120 (ENB2) is represented in FIG. 2 as a rectangle 220. The rectangle 220 may also be referred to as the bandwidth 220. The bandwidth frequency in the diagram increases from the bottom of rectangle 220 to the top of rectangle 220.
In the Fractional Frequency Reuse (FFR) method, the bandwidth 210 and the bandwidth 220 are both divided into a number of sub-bands. A scheduler (not shown in FIG. 2) in each of the wireless base stations schedules the sub-bands of its respective bandwidth to the mobile units that are in wireless communication with the wireless base station. For example, the scheduler of the first wireless base station 110 may assign the sub-band of the bandwidth 210 that is designated with the reference numeral 230 to the mobile unit 130. The scheduler of the second wireless base station 120 may also assign the sub-band of the bandwidth 220 that is designated with the reference numeral 240 to the mobile unit 130. In this manner bandwidth resources are assigned to the mobile unit 130 at both of the wireless base stations 110 and 120.
The scheduler of the first wireless base station 110 assigns other sub-bands of the bandwidth 210 to other mobile units (not shown). The scheduler of the second base station 120 assigns other sub-bands of the bandwidth 220 to other mobile units (not shown). Each of the multiple frequency sub-bands may have a different effective spectral efficiency. The scheduler determines the bandwidth fractions for all of the mobile units in each frequency band in order to try to maximize the net sum of user utilities. User utilities are functions of the average rates of the users, where different averaging rules can be used for different users.
There is a need in the art for a frequency allocation scheduler that is able to maximize the net sum of user utilities by computing optimal bandwidth fractions for all the users in each frequency band.